Field of the Invention
This disclosure relates to bias tees, and, more particularly to configurable bias tees that improve current processes for low current measurements and AC high impedance measurements.
Description of Related Art
A bias tee is typically a passive, three-port electrical network that may act as a diplexer. In one mode of operation, one port of the bias tee network is connected to a very low frequency or direct current (DC) source and another port is connected to a high frequency or alternating current (AC) source. The bias tee combines the DC source signal and the AC source signal so that the third port of the network is simultaneously coupled to both the DC and AC signals. Bias tees are well-known electrical devices which are useful in many applications where it is necessary to inject DC power into an AC signal. Typical applications include powering photodiodes, lasers, or remote antenna amplifiers.
Bias tees are also typically bi-directional. Therefore, in another mode of operation, a combined AC and DC (“AC+DC”) signal is applied to the third port of the tee, and the bias tee network separates the AC and DC components of the signal so that the AC component of the signal can be measured at the AC port of the tee, and the DC component of the signal can be measured at the DC port of the tee. Examples of applications that use a bias tee in this mode include packaged device characterization and wafer probing. In these types of applications, connecting the combined AC+DC port of the tee to the output of the device under test allows a user to measure the DC characteristics of the device, and to measure the AC characteristics of the device, without having to re-configure the test setup between the DC and AC tests. In such applications, for certain types of devices, the bias tee carries very low DC current levels, as well as AC signals for high impedance measurements to the measurement instrument. Achieving good performance for both low current DC measurements and AC high impedance measurements presents special challenges to the designer of a bias tee.
The simplest bias tee designs employ a capacitor, a resistor, and three coaxial connectors. The coaxial connectors serve as a DC signal port, an AC signal port, and a combined AC+DC signal port for the tee. The capacitor is connected between the AC signal port and the AC+DC signal port. The resistor is connected between the DC signal port and the AC+DC signal port. The overall DC performance of this bias tee design is limited because the resistive element limits the current that can travel through the DC path of the tee.
Improved DC performance is achieved with a modified bias tee design in which the resistor in the DC path is replaced with an inductor. Although an ideal inductor would block the AC signal from passing back to the DC port, the AC performance of this design can be limited by the potential LC resonance effects. Also, since such a design uses coaxial connectors as the ports of the tee, its low current performance is limited due to the leakage current inherent in coaxial connectors.
To improve low current performance, triaxial connectors, rather than coaxial connectors, are used for the DC port and the AC+DC port. The single capacitor in the designs described above is replaced with two capacitors in series. One of the capacitors is “guarded” by the DC signal, thereby minimizing the leakage current through this capacitor. However, because this capacitor usually has relatively large capacitance, it will tend to generate current noise, thereby still hampering the low current performance of the bias tee.
Embodiments of the invention address these and other limitations of the prior art.